A flow rate of refrigerant which circulates through a refrigeration cycle apparatus is all the same in any points in a refrigeration cycle. If a suction density of refrigerant passing through a compressor is defined as DC and a suction density of refrigerant passing through an expander is defined as DE, the DE/DC (density ratio) is always constant.
In recent years, attention is focused on a refrigeration cycle apparatus using, as refrigerant, carbon dioxide (CO2, hereinafter) in which ozone destroy coefficient is zero and global warming coefficient is extremely smaller than Freon. The CO2 refrigerant has a low critical temperature as low as 31.06° C. When a temperature higher than this temperature is utilized, a high pressure side (outlet of the compressor—gas cooler—inlet of pressure reducing device) of the refrigeration cycle apparatus is brought into a supercritical state in which CO2 refrigerant is not condensed, and there is a feature that operation efficiency of the refrigeration cycle apparatus is deteriorated as compared with a conventional refrigerant. Therefore, it is important for the refrigeration cycle apparatus using CO2 refrigerant to maintain optimal COP, and if an operating condition is changed, it is necessary to obtain an operating state (pressure and temperature of the refrigerant) which is optimal to this operating condition.
However, when the refrigeration cycle apparatus is provided with the expander and power recover by the expander is used as a portion of a driving force of the compressor, the number of rotation of the expander and the number of rotation of the compressor must be the same, and in the expander which is designed optimally with a predetermined density ratio, it is difficult to maintain the optimal COP when the operation condition is changed.
Hence, there is proposed a structure in which a bypass pipe which bypasses the expander is provided, the refrigerant amount flowing into the expander is controlled, and the optimal COP is maintained (see patent documents 1 and 2 for example)
[Patent Document 1]
Japanese Patent Application Laid-open No. 2000-234814 (paragraphs (0024) and (0025) and FIG. 1)
[Patent Document 2]
Japanese Patent Application Laid-open No. 2001-116371 (paragraph (0023) and FIG. 1)
However, there is a problem that as a difference between an amount of refrigerant which flows into the expander and an optimal flow rate in terms of design is increased, an amount of refrigerant flowing through the bypass pipe is increased and as a result, power which could have been recovered can not sufficiently recover.
If the power recover by the expander is used as a driving force for an auxiliary compressor which is different from the compressor, it is possible to eliminate the constraint that the number of rotation of the expander and the number of rotation of the compressor must be the same. However, even if the auxiliary compressor is driven by the expander, the constraint that the density ratio is constant is still remained, and it is still necessary to control the amount of refrigerant which flows into the expander.
Thereupon, it is an object of the present invention to reduce the constraint that the density ratio is constant as small as possible, and to obtain high power recovering effect in a wide operation range.
It is another object of the invention to introduce high pressure refrigerant in a halfway of the expansion process to increase the flow rate of refrigerant per one expansion process, thereby recovering power efficiently.